In recent years, not only has the applicability of magnetic recording devices such as a magnetic disk device, flexible disk device, or magnetic tape device been remarkably broadened, and its importance has been increased, but the recording density of a magnetic recording medium used in such a device has also been remarkably improved. In particular, the area density has been strikingly increased since an MR head and the PRML technique were introduced. Recently, a GMR head, TMR head, etc. have been introduced, and the area recording density has been continuously increased at the rate of about 100% per year. Achievement of higher recording density is still sought in developing such a magnetic recording medium. Therefore, a higher coercive force, a higher signal-to-noise ratio (SNR), and higher resolution are required in the magnetic recording layer. Additionally, in recent years, considerable efforts have been devoted to increasing not only the track recording density but also the track density in order to increase area recording density.
The track density of the latest magnetic recording device reaches about 110 kTPI. However, as the track density is increased, magnetically-recorded data will interfere with each other between adjacent tracks. Consequently, the magnetic transition region of the boundary portion will be a noise source, and it is likely to impair the SNR. This leads to a increase of the bit error rate, and such a problem is an obstacle to improving the recording density.
In order to increase the area recording density, it is necessary to achieve the largest possible saturation magnetization and magnetic thickness in each recording bit on the magnetic recording medium by way of adjusting the size of each recording bit to be smaller. However, as the recording bit is made smaller, the magnetic minimum volume per bit will be small, and there is a problem in which recorded data disappear due to the flux reversal caused from heat fluctuation.
Furthermore, because the distance between tracks is narrower, such a magnetic recording device requires an extremely precise track servo technology, and also, a technique is generally adopted, in which recording is broadly done, and reading is done narrower than recording in order to exclude the influence from adjacent tracks as much as possible. In this method, the influence between tracks can be minimized. However, it is difficult to attain sufficient reading output in the method, and there is a problem in which it is difficult to obtain a sufficient SNR.
An example of a method which attempts to solve the problem of the heat fluctuation, to attain a sufficient SNR, or to attain a sufficient output can be mentioned. In this method, an attempt is made to increase the track density in the following way. That is, a projected part and a recessed part are formed on the surface of the magnetic recording medium along tracks to physically separate recording tracks from each other, thereby increasing the track density. Such a technique is called the “discrete track method”, and the magnetic recording medium produced by the method is called as a “discrete track medium”.
As an example of such a discrete track medium, a magnetic recording medium is known, where the magnetic recording medium is formed on a non-magnetic substrate having a projected and recessed pattern on its surface, and physically-separated magnetic recording tracks and a servo signal pattern are formed therein (For example, see Patent Document 1).
In this magnetic recording medium, a ferromagnetic layer is formed through a soft magnetic layer over the surface of the substrate having a plurality of projected and recessed parts thereon, and a protective layer is further formed on its surface. In the magnetic recording medium, a magnetic recording region is formed in the projected part, and the magnetic recording region is physically divided from the surroundings thereof. According to the magnetic recording medium, generation of a magnetic domain wall can be prevented in the soft magnetic layer. Consequently, the influence of heat fluctuation is negligible, and interference between adjacent signals does not occur. Therefore, it is considered that a high density magnetic recording medium which exhibits less noise can be produced.
The discrete track method includes the following two techniques. That is, a technique wherein tracks are formed after a magnetic recording medium including several thin layers is formed; or a technique wherein thin layers of the magnetic recording medium are formed directly on the surface of a substrate in advance or after a recessed and projected pattern is formed on the thin layers for the track-formation can be mentioned (for example, see Patent Document 2 or 3). The former technique is often called “magnetic layer-processing type”. The technique has a disadvantage in which the medium is likely to be contaminated during the production process because physical processing is conducted on its surface after forming the medium, and the production process is very complicated. On the other hand, the latter method is often called an “embossing type”, and contamination during its production process hardly occurs. However, the recessed and projected shape formed on the substrate will pass into layers formed thereon. Therefore, there is a problem in which the flying position and the flying height of a recording/reading head, which record or read data while flying over the medium, are not stable.
Furthermore, another method is disclosed (see Patent Documents 4 to 6). In the method, ions such as nitrogen or oxygen are injected into a magnetic layer formed in advance, or a laser beam is irradiated to the magnetic layer whereby magnetic properties of the treated portion are modified to form regions between the magnetic tracks of the discrete track medium.
However, the ion injection or the laser irradiation damages the magnetic layer in this method, and a projected and recessed structure is often formed on the surface of the magnetic layer. Additionally, the energy density is low with respect to the entire surface of the medium in this method although the injected ions or the laser has high energy, and there is a problem in which this requires a long-term treatment to modify the magnetic properties of the entire surface of the medium.
In addition, Patent Document 7 discloses a patterning method for a magnetic material wherein an exposed area of the surface of a ferromagnetic layer of a magnetic recording medium is exposed to an active reaction gas containing a halogen to fluorinate the ferromagnetic material whereby the ferromagnetic material is formed into a non-ferromagnetic body.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-164692
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2004-178793
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2004-178794
Patent Document 4: Japanese Unexamined Patent Application, Publication No. H5-205257
Patent Document 5: Japanese Examined Patent Application, Publication No. 2006-209952
Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2006-309841
Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2002-359138